Types of wireless networks include infrastructure-based wireless networks and ad hoc wireless networks.
Ad hoc networks are self-forming networks which can operate in the absence of any fixed infrastructure, and in some cases the ad hoc network is formed entirely of mobile nodes. An ad hoc network typically includes a number of geographically-distributed, potentially mobile units, sometimes referred to as “nodes,” which are wirelessly connected to each other by one or more links (e.g., radio frequency communication channels). The nodes can communicate with each other over a wireless media without the support of an infrastructure-based or wired network. Links or connections between these nodes can change dynamically in an arbitrary manner as existing nodes move within the ad hoc network, as new nodes join or enter the ad hoc network, or as existing nodes leave or exit the ad hoc network. Because the topology of an ad hoc network can change significantly techniques are needed which can allow the ad hoc network to dynamically adjust to these changes. Due to the lack of a central controller, many network-controlling functions can be distributed among the nodes such that the nodes can self-organize and reconfigure in response to topology changes.
One characteristic of ad hoc network nodes is that each node can directly communicate over a short range with nodes which are a single “hop” away. Such nodes are sometimes referred to as “neighbor nodes.” When a node transmits packets to a destination node and the nodes are separated by more than one hop (e.g., the distance between two nodes exceeds the radio transmission range of the nodes, or a physical barrier is present between the nodes), the packets can be relayed via intermediate nodes (“multi-hopping”) until the packets reach the destination node. In such situations, each intermediate node routes the packets (e.g., data and control information) to the next node along the route, until the packets reach their final destination. For relaying packets to the next node, each node maintains routing information collected through conversation with its neighboring nodes. The routing information can also be periodically broadcast in the network to reflect the current network topology. Alternatively, to reduce the amount of information transmitted for maintaining accurate routing information, the network nodes may exchange routing information only when it is needed. One approach for routing information, known as Mesh Scalable Routing (MSR), is described in United States Patent Application Publication Number 20040143842 entitled “System And Method For Achieving Continuous Connectivity To An Access Point Or Gateway In A Wireless Network Following An On-Demand Routing Protocol, And To Perform Smooth Handoff Of Mobile Terminals Between Fixed Terminals In The Network,” filed Jan. 13, 2004, which is incorporated by reference herein in its entirety.
Carrier Sense Multiple Access (CSMA) is a probabilistic Media Access Control (MAC) protocol in which a node verifies the absence of other traffic before transmitting on a shared physical medium, such as an electrical bus, or a band of electromagnetic spectrum. “Carrier Sense” describes the fact that a transmitter node listens for carrier wave before trying to send. The transmitter node tries to detect the presence of an encoded signal from another node before attempting to transmit. If a carrier is sensed, the transmitter node waits for the transmission in progress to finish before initiating its own transmission. “Multiple Access” means that multiple nodes send and receive on the medium.
In a pure CSMA network, only the carrier sense is used to avoid collisions. Concurrent transmission by multiple nodes can result in frame collisions. For example, if two nodes try to send a frame concurrently (e.g., at nearly the same time), neither detects a carrier and therefore both begin transmitting. In this situation, the concurrent transmissions interfere with each other so that receiver nodes are unable to distinguish the overlapping received signals from each other. Receiver nodes cannot distinguish between collisions and other sources of frame errors, and therefore collision recovery relies on the ability of the communicating nodes to detect frame errors and invoke an error recovery procedure.
In CSMA networks it is generally not possible to entirely prevent collisions. CSMA variants developed to address collisions include, for example, Carrier Sense Multiple Access With Collision Avoidance (CSMA/CA), and Carrier Sense Multiple Access With Collision Detection (CSMA/CD).
In CSMA/CA, each node informs other nodes of its intent to transmit. When the other nodes have been notified, the information is transmitted. This arrangement prevents collision because all nodes are aware of a transmission before it occurs. However, collisions are still possible, and are not detected, so they have the same consequences as in pure CSMA.
In CSMA/CD, sending nodes are able to detect when a collision occurs and stop transmitting immediately, backing off for a random amount of time before attempting to transmit again. This results in much more efficient use of the media since the bandwidth for transmitting the entire frame is not wasted. However, it is not possible with all media (e.g., radio), and requires extra electronics.
CSMA MAC protocols work well with bursty, non-periodic traffic such as http, web surfing, and the like, but can incur significant delays for time sensitive, periodic traffic.
Time Division Multiple Access (TDMA) is a technology for shared medium networks. TDMA MAC protocols require time synchronization and slot reservation for collision free transmission. TDMA MAC protocols allow several users to share the same frequency by dividing it into different timeslots. The users transmit in rapid succession, one after the other, each using their assigned timeslot. This allows multiple users to share the same transmission medium (e.g., radio frequency) while using only the part of its bandwidth which they require.
TDMA MAC protocols are generally regarded as being efficient for periodic, delay sensitive traffic (e.g., voice traffic and video traffic), since they provide contention free transmission. On the other hand, as described previously herein, CSMA MAC protocols work well with bursty traffic (e.g., web surfing).
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